MOEPHOLOGY AND EVOLUTION
Lower Molar Teeth—
Antero-external cusp .........protoconid (PED).
Antero-internal cusp .........metaconid (MD).
Postero-esternal cusp.........liypoconicl (HD).
Postero-internal cusp .........entoconid (END),
Postero-intermediate Gusp...Jiypoconi(lid (HLD).
In Cebut the three upper molar teeth diminish in size in the order 1, 2, 3, and M3 is minute. The four cusps are present, and the protocones are connected to the metacones by oblique ridges in some species. The three lower molar teeth also diminish in the order 1, 2, 3. The hypocones are insignificant, especially on the degenerate third tooth. Faint ridges connect the para-cones, protocones and inetacones. In Callicebus, the most primitive Platyrrhine, the front of the protocones have sharp crests ; and they are connected behind to the hypocones, which are really pseudohypocones. InAotus the hypocones are well marked. In Aloitatta, which is peculiar among the Platyrrhini for its leaf diet, the teeth are highly specialized.
In the Hapalidse the third molar teeth have been lost. But the remaining ones have three low, rounded cusps, and the crowns themselves are rounded.
Vertebral Column.—The spinous processes of the dorso-lumbar vertebrae converge to a centre of motion, which varies in position, but is always in the lower dorsal region. The cervical vertebrae are seven in number in all species. The transverse processes have anterior ridges, or are triangular as in lower Mammalia, and the axis vertebra has a strong spinous process. The number of dorsal vertebrae and ribs is 13 or 14, of which 10 are sternal ribs. The lumbar vertebrae are 5 to 8 in number,